This invention pertains to devices and methods of making devices having overstress indicators thereon. This invention is beneficial to devices which undergo stresses during manufacturing and testing processes and during general use. More particularly, the invention pertains to a computer system or a subassembly thereof, and methods of making such, wherein visual indication of stress is provided by simple inspection.
Overstrain caused by overstress to solder joints on Ball Grid Array (BGA) and other high pin-count electronic packages during manufacturing and customer usage has caused serious premature system failure and customer impact on systems ranging from servers to laptop computers. Overstress conditions are usually caused by insufficient mechanical support in in-circuit testing (ICT), card functional testing, system integration, and assembly. For mobile computer applications, rough handling by customers can also induce this type of failure to solder joints. Typically, overstress to the BGA part is evident from hairline cracks between the solder ball and the chip substrate or between the solder ball and the printed circuit board (PCB). In general, the damage to solder joints produces microcracks at the inter-metallic interfaces which are largely undetectable in normal manufacturing test processes.
A number of advanced non-destructive methods such as eddy-current tests, electric current tests, and ultrasonics, among others, are available for detecting the surface crack. The eddy-current test can only be applied to conductive materials and masked or false indications are possible due to sensitivity to part geometry and permeability variations. The electric current test is sensitive to surface contamination, edge effects and electrode spacing. Ultrasonic techniques are difficult when applied to small, thin, complex parts and require special probes. Trained operators, specialized equipment, increased complexity and cost, and the ability to automate the process are among other disadvantages of these methods.
One destructive test method, such as a dye penetration method, can also be used. In this approach a red dye is injected at the solder joints of the BGA package and the BGA is pulled off the board and the cross section site is inspected under microscope for evidence of solder joint crack. A similar approach involves taking cross-sectional cuts through the BGA package and examining these encapsulated metallurgical samples for evidence of cracking using a scanning electron microscope or other high magnification microscope in the range of 2000×. These destructive approaches are time-consuming, expensive, and require highly specialized equipment and personnel.
Clearly there is a need for a simpler approach to detect these microcracks, an approach that avoids the disadvantages of the current methods.